When Prisoner 106 arrived at the maximum security facility of Level Six in the New Mexico State Penitentiary in Santa Fe, he was on the books as a dangerous armed robber headed straight for solitary confinement.

But as he was moved into his 12-foot-by-7-foot cell, where he would have little to no physical contact with the outside world, the thing that almost no one at the prison knew was that this inmate was actually the Secretary of Corrections for the state of New Mexico.

Gregg Marcantel is a former cop who spent a career putting away criminals, and he decided to put himself in solitary in Level Six to live among the worst of the worst inmates.

“This is where our predators, our monsters, are dangerous inmates are housed,” said Alex Tomlin, the public affairs director for New Mexico's Department of Corrections. “My greatest fear is that something happens to [Marcantel].”

“[Even] 24 hours in a maximum security prison is still a lot of time,” she continued. “The next 48 hours will really just be about waiting and praying.”

There are an estimated 80,000 inmates locked up in solitary confinement in the United States, but this form of incarceration has become extremely controversial. Not only is it three times as expensive to house an inmate in solitary, but critics say it can also make inmates more dangerous.

Marcantel said he was doing this because he wants to reform the state's use of solitary. He said most of the inmates who are placed in solitary confinement will end up back on the streets, transformed into super criminals.

ABC News

PHOTO: Gregg Marcantel, the Secretary of Corrections for the state of New Mexico, spent 48 hours undercover in solitary confinement at the New Mexico State Penitentiary, outside of Santa Fe, New Mexico.

“We are sending them back to our neighborhoods worse than when they came... I can’t allow that,” he said.

But before he made any changes, Marcantel wanted to experience solitary from the inside, so he spent 48 hours living in lockdown in Level Six.

Level Six is divided up into three separate housing units, holding a total of 282 inmates, who are doing time for crimes that range from rape to armed robbery to murder. Combined, the men here have killed 138 people.

Once in his cell, Marcantel sorted through his prison-issued personal effects and opened a letter from his wife.

“The whole time we’ve ever been together, there’s never been a day that we didn’t speak to one another,” he said.

Very quickly, the inmates started to buzz about the latest arrival, yelling “hey, new guy” and “Cell 6, who the [expletive] are you?” Correction officers told the inmates that Prisoner 106 was speech and hearing impaired because the secretary worried that if the inmates hear his distinctive voice, his cover will be blown and his security compromised.

As the hours rolled by, the secretary started to realize that the greatest danger wasn’t from other inmates, but from his own mind.